A large and growing population of users enjoys entertainment through the consumption of digital media items, such as music, movies, images, electronic books, and so on. Users employ various electronic devices to consume such media items. Among these electronic devices are electronic book readers, cellular telephones, personal digital assistants (PDAs), portable media players, tablet computers, netbooks, and the like. These electronic devices wirelessly communicate with a communications infrastructure to enable the consumption of the digital media items. Some of these types of electronic devices include an electro-optic display, such as electro-optic displays having encapsulated electrophoretic media. These media typically have internal cavities which contain a fluid (either liquid or gas). Such electro-optic displays may include encapsulated electrophoretic displays, encapsulated liquid crystal displays, and other type of display as described herein.
Electro-optic displays include a layer of electro-optic material, referring to a material configured to operate in first and second display states differing in at least one optical property, the material being changed from its first to its second display state by application of an electric field to the material. Although the optical property is typically color perceptible to the human eye, it may be another optical property, such as optical transmission, reflectance, luminescence or, in the case of displays intended for machine reading, pseudo-color in the sense of a change in reflectance of electromagnetic wavelengths outside the visible range. For example, bistable, multi-stable types of displays may be used.
Electrophoretic displays (EPD), however, are sensitive to moisture (both ingress and egress) and ultraviolet (UV) light. Conventional EPDs utilize a top-hat protective layer with an ‘underfill’ edge seal to form a moisture barrier. FIG. 1 illustrates a conventional EPD 100 with an underfill edge seal 126. The conventional EPD 100 includes the following layers: a backplane 106, a layer of electro-optic medium 110, a conductive layer 14, a substrate 12, and a protective layer 112. The layer of electro-optic medium 110 is a material configured to operate in first and second display states differing in at least one optical property, the material being changed from its first to its second display state by application of an electric field to the material. Although the optical property is typically color perceptible to the human eye, it may be another optical property, such as optical transmission, reflectance, luminescence, or pseudo-color. The electro-optic medium 110 may be encapsulated electrophoretic, encapsulated liquid crystal displays, and other types of electro-optic displays as would be appreciated by one of ordinary skill in the art having the benefit of this disclosure. The electro-optic medium 110 may include multiple microcapsules, each of which includes a capsule wall containing a hydrocarbon-based liquid in which negatively-charged white particles and positively-charged black particles are suspended. Upon application of an electrical field across the medium 110, the white particles move to positive electrode and the black particles move to the negative electrode to display either white or black on the display. In particular, the backplane 106 has at least one pixel electrode, such that application of an electrical potential between the conductive layer 14 and the pixel electrode can change the optical state of the electro-optic medium 110. The backplane 106 includes at least one contact pad electrically isolated from the pixel electrode, and the conductive layer 14 is electrically connected to the contact pad through a conductive via extending from the conductive layer 14 through or past the electro-optic medium 110 to the contact pad. The substrate 12 is a light transmissive substrate, such as a transparent plastic film, such as Polyethylene terephthalate (PET) film, sometimes written poly(ethylene terephthalate). The substrate 12 carriers a thin light-transmissive electrically-conductive layer 14 (e.g., indium tin oxide (ITO)), which acts as the front electrode in the final display. The protective layer 112 may be a protective film laminated over the front plane laminate in the same lamination operation by which the front plane laminate is laminated to the backplane 106. Such a protective film can protect the electro-optic medium against ingress of moisture, other liquids, and some gases. The protective layer 112 absorbs ultra-violet radiation, and the protection layer 112 (and other barrier layers) can be used prevent ingress of oxygen or moisture into the final display and provide anti-reflection coatings to improve the optical properties of the final display.
In the depicted display 100, the protective layer 112 extends beyond the periphery of the electro-optic medium, and a sealing material 126 is placed around at least part of the periphery of the electro-optic medium between the backplane 106 and the conductive layer 14, substrate 12, and protective layer 112. The protective layer 112, substrate 12 and the conductive layer 14 are all larger than the layer of electro-optic medium 110 by a distance of about 0.5 to 1.5 mm. The overhang of the protective layer 112, substrate 12 and conductive layer 14 may be provided by sizing the layer of electro-optic medium 110 to have a smaller surface area than the other layers, or by removing a peripheral portion of the layer of electro-optic medium 110 from around the periphery of the display prior to the lamination of the layers to the backplane 106. Also, prior to lamination, sealing material is dispensed around the periphery of the display so that when the layers are laminated to the backplane 106, the edge seal 126 is formed. Alternatively, the sealing material may be applied after lamination by using capillary forces or direct pressure to fill the sealant into the small cavity around the electro-optic medium 110.
As described above, the protective layer 112 can protect the electro-optic medium against ingress of moisture, other liquids, and some gases. However, even with such a protective layer 112, the edge of the electro-optic medium 110 is still exposed to the environment. For this, the display 100 includes the edge seal 126, which serves to prevent the ingress of moisture and other contaminants around the outer edges of the display. The underfill edge seal is typically a curable sealant material that is applied around the outside edge of the protective layer 112, and wicks beneath it to form a moisture barrier. However, this wicking process generally only works reliably over a short distance, for example, 1-2 mm, limiting the degree of overhang of the protective layer 112 over the edge of the electro-optic medium 110.
FIG. 2 illustrates a conventional double adhesive seal electro-optic display 200 with an additional front protective layer 218. The display 200 includes a backplane 202, a lamination adhesive layer 204, a layer 206 of electro-optic material, a second lamination adhesive layer 220, and a front substrate 208. The electro-optic layer 206 is sandwiched between a backplane 202 and a protective layer 218 and a sealing material extends between the backplane 202 and the protective layer 218. The electric-optic display 200 also includes another adhesive layer 228 between the protective layer 218 and the front substrate 208. The protective layer 218 is a top-hat protective layer 218 that extends beyond an edge of the other layers (208, 220, 206, 204). The dimensions of the top-hat protective layer 218 are less than the backplane 202 to allow the sealing material to be disposed between the protective layer 218 and the backplane 202. For example, the top-hat protective layer 218 may be 1-2 mm less than the backplane 202. The protective layer 218 is secured to the backplane 202 or to a second protective layer adjacent the backplane 202. The electro-optic layer 206 can also be sealed between two layers of adhesive or between one layer of adhesive and the backplane 202. The display 200 also includes a bead of sealing material that extends from the backplane 202 to the rear surface of the protective layer 218, thus forming an underfill edge seal 216. The display 200 utilizes the top-hat protective layer 218 with the ‘underfill’ edge seal 216 to form a moisture barrier. The underfill edge seal 216 can be formed by dispensing around the periphery sealing material and being laminating to the backplane 202. Alternatively, the sealing material may be applied after lamination by using capillary forces or direct pressure to fill the sealant into the small cavity around the electro-optic medium 206.
While it is potentially possible to attach a larger cover sheet to the top of the existing protective layer (112, 218), there are several reasons not to take this approach. First, the protective layer (112, 218) is a costly multi-layer stack, which adds several dollars to the cost of the assembly. Second, the protective layer (112, 218) is unnecessarily thick, which would increase the z-height of the finished display and thus of the device. Most device developers generally aspire to make their devices as thin as possible to match consumer expectations. Finally, the protective layer (112, 128) both absorbs and scatters light, decreasing the brightness and contrast of the display.